This invention relates to electro-optical display devices in general and, more particularly, to display panels employing electrophoretic dispersions for producing graphic data.
The electrophoretic effect is well known and the prior art is replete with a number of patents and articles which describe the effect. As will be recognized by a person skilled in the art, the electrophoretic effect operates on the principle that certain lo particles, when suspended in a medium, can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrophoretic image displays (EPIDs) utilize the electrophoretic effect to produce desired images. In prior art EPIDs colored dielectric particles are suspended in a fluid medium that is either clear or an optically contrasting color as compared to the dielectric particles. The colored electrophoretic particles are then caused to selectively migrate to, and impinge upon, a transparent screen electrode, thereby displacing the fluid medium from the screen and creating the desired image.
For a suitable example of such devices using the electrophoretic effect, reference is made to U.S. Pat. No. 4,732,830 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS and issued to Frank J. DiSanto et al. on Mar. 22, 1988. In this patent, there is disclosed an electrophoretic display apparatus which includes a planar transparent member having disposed thereon a plurality of vertically extending, electrically conductive lines defining a grid. A plurality of horizontally extending, electrically conductive cathode lines are disposed on top of the vertical lines but are insulated therefrom by a thin insulating layer, thereby forming an XY matrix of electrodes. A conductive plate or anode is spaced above the line patterns and disposed therebetween is an electrophoretic dispersion of yellow pigment particles in a dark colored suspension medium. The particles are transportable within the suspension medium under the influence of an electric field, by selectively biasing the conductive lines or the conductive plate to attract them.
A variety of pigment particle and dispersion medium compositions are known in the art. See, for example, U.S. Pat. No. 4,298,448 entitled ELECTROPHORETIC DISPLAY issued on Nov. 3, 1981 to K. Muller et al. This patent describes an electrophoretic display which utilizes electrophoretic particles of various pigments dispersed in a suspension consisting of liquid paraffin, 1,2-trichlorotrifluoroethane, and a solvent dye. The pigments are coated with an organic material which contains a charge control agent to cause the particles to possess a uniform surface potential and thus allow the particles to move in a more controlled manner.
U.S. Pat. No. 4,680,103, entitled POSITIVE PARTICLES IN ELECTROPHORETIC DISPLAY DEVICE COMPOSITION issued on Jul. 14, 1987 to Beilin Solomon I et al. describes a suspension for an EPID system in which the pigment particles are coated with a organosilane derivative and dispersed within an aprotic solvent such as alkyl and aromatic nitriles, dialkysulfoxides, alkyl phosphoric triamide, dimethylformamide, nitroalkane, or mixtures thereof.
As can be gathered from an inspection of the aforementioned references, the plane containing the horizontally extending conductive lines is closely spaced to that containing the vertically extending conductive lines, thereby forming an X-Y matrix of insulated grid and cathode electrode elements. The aforementioned planes may, in fact, be three microns apart or less. Because of this close spacing, the application of potential differences between the grid and cathode elements can generate electric fields of great intensity. These fields can exceed five million volts per meter. The aggregate effect of these fields can be surprisingly large due to the great length of electrode lines present in the display. For example, for a grid electrode which has 1,280 four-inch lines, each line being divided into six tynes and each tyne having two edges, the effective edge-length of the grid lines is about one-mile. In addition, these tynes have a certain degree of microscopic and submicroscopic unevenness which results in local increases in field strength.
The above noted combination of intense field and many edges promotes the occurrence, on a continuous basis, of transient mini-arcs between the cathode and grid lines. Such transient electrical discharges can cause the breakdown of the dielectric dispersion medium used in the electrophoretic display. While the liquid quickly seals off the breakdown, there are chemical by-products left from the mini-arc. For a discussion of the electrical breakdown process of dielectric fluids generally, reference is made to an article by Y. Yamada et at. entitled "Studies of the Breakdown Process in Dielectric Liquids Using High Speed Photography", Journal of Electrostatics, Vol. 7 (1979), pp. 155-168.
Within the context of electrophoretic display suspensions, it will be readily appreciated that a variety of chemical reactions may occur as a result of the aforementioned electrical arcing. As suggested by the Yamada reference, for example, the breakdown of the solvent molecules is a highly probable result. In a typical EPID suspension containing carbon tetrachloroethylene, for example, the presence of hydrogen and chlorine in atomic and molecular form, or even the highly stable hydrogen chloride molecule, may be attributed to electrical discharges therein. Over time, these gases collect and produce bubbles in the display. J. H. Hildebrand et at. suggest that such gasses are highly soluble in non-polar solvents such as those used in electrophoretic display suspensions. See J. H. Hildebrand et al, Viscosity and Diffusivity, Chapter 7, pp. 49-66, John Wiley and Sons, New York (1977). However, although the solubility of the solvent may delay the appearance of gas bubbles as the breakdown process continues, the bubbles will eventually appear and thereby shorten the useful life of the electrophoretic dispersion.
Accordingly, it is an object of the present invention to provide a superior electrophoretic suspension having an operating life which is much greater than those utilized in conventional electrophoretic displays. It is a further object of the present invention to provide an electrophoretic display which advantageously utilizes such a suspension and which is both economical to fabricate and reliable to operate.